Infinite Solitude (stylized as InfiniteSolitude in the text) is a keyword ability that activates when the card with this ability appears on Square. As of now, this ability is only possessed by "Twin Star Regalix, Cyclotron".
The current explanatory text of Infinite Solitude ability is shown below:
[Auto]Infinite Solitude (When this card appears, choose any number of your Z/X on Square except this card, and return them to hand.)
Comprehensive Rules[]
1109 Infinite Solitude
1109.1 Infinite Solitude ability is an automatic ability that returns your own Z/X on Square to hand from the appearance of a card.
1109.2 "Infinite Solitude" means " <Valid> Square <Trigger> This card appears. <Effect> Choose any number of your Z/X on Square excluding this card, and return them to your hand."
1109.2a You can opt to choose 0 amount of card, and not return any cards to hand as a result.